Field of Invention
The present invention relates to improvements in temperature limiters intended to ensure safety of any heating system and in particular of a halogen and/or radiant cooking element whose heating power is regulated by an energy measurer or by any other system, as it is in that case that its application appears to present the greatest interest.
French Patent 1 204 872 describes an alarm device for thermal engine based on the sudden variation of the magnetic permeability (Curie point) of a pellet made of an alloy of appropriate composition and which cooperates with an elastically biased movable element in combination with a magnet so as to control opening of the contacts of an electric circuit. The temperature limiter according to the French Patent mentioned above presents a major drawback concerning the nature of the materials used. In fact, the tube containing the movable element is made of stainless steel, so that its expansion is detrimental to the precise release of the apparatus at the desired temperature. In fact, the greater the distance between the magnet and the pellet of metal alloy, the more the force of attraction decreases, so that the magnet is released more rapidly from the pellet. This results in a reduction in the rapidity of release and re-locking, as the force of the spring approaches the force of attraction between the magnet and the pellet. Moreover, the stainless steel tube does not guarantee electrical insulation between the metal parts and the live cooking element.
The device according to the French Patent in question presents a second drawback in that the magnet is placed at the end of the device, therefore at the spot where the temperature is the highest. Now, the limit of use of permanent magnets best resisting temperature is of the order of 500.degree. C. Therefore, under these conditions, the magnet will rapidly lose its magnetic permeability, therefore its force of attraction. Finally, the device according to French Patent 1 204 872 is a safety device and not a temperature limiter which must function a large number of times around the temperature limit.
Limiters are also known which are in the form of a probe disposed in the vicinity of lamps or resistors and which are constituted by two elements, a glass tube which virtually does not dilate and a metal rod dilating inside the tube. The relative position of the two elements must be adjusted in order to obtain release at the correct temperature, which process is long and expensive. Moreover, the product is fragile.